There is a need in industry for separating fine particles according to their size, shape and/or density. In countercurrent solid-liquid extraction processes, it is necessary to move particles and liquid in opposite directions. Some examples of particle fractionation are cleaning of coal slurries where heavier pyrite particles containing sulphur are removed from coal, fractionation of chromatography packing materials into narrow size ranges in order to limit axial dispersion of the species to be separated, and separation of ceramic materials into fractions with narrow particle size distributions to enhance strength and uniformity of sintered parts. Examples of countercurrent extraction are decaffeination of coffee beans, hydrometallurgical extractions, and extraction of edible oils from oil seeds using hexane.
The prevalent methods used for particle size fractionation are screening, centrifugal sedimentation and flotation. Each of the above methods is best suited to a different range of particle sizes and types with considerable overlap among them. As the particle size decreases below 100 .mu.m, separation becomes difficult. Most physical separation techniques lose their effectiveness in this range and surface chemical differences or particle motion in an induced field such as a centrifugal or magnetic field becomes the basis for separations. In general, only high speed centrifuges are effective for separations below 10 .mu.m. However, their costs are high due to their mechanical complexity.
Sharp separations and high product purities are rarely achieved because all the above techniques are inherently single stage separation operations. Staging involves installing additional hardware with associated increases in capital and operating costs. Recycling is difficult, and for centrifugal separations, it is energy intensive. Hence in most processes only a few stages are employed. In addition, the flexibility of these classical methods is somewhat limited; changes in particle characteristics often require hardware modifications.
Two more modern separation techniques are of especial interest since they are more closely related to the present invention than the general methods discussed above. These techniques are parametric pumping and field-flow fractionation.
The process known as parametric pumping is a relatively new concept in separation processes. The technique is generally credited to Wilhelm and his coworkers although the method was apparently conceived earlier under the name "Heatless Distillation" by Skarstrom. In this technique cyclic changes in some intensive variable like temperature, pressure, or pH periodically shift the equilibrium distribution of solute between two phases. The modest composition differences induced are amplified and organized into useful separations by coupled, cyclic, bi-directional flows. In parametric pumping the cyclic flow change and intensive variable change are out of phase.
The number of stages in a small apparatus can be very large and dramatic separations and purities of adsorbed species can be achieved. The main limiting factor of parametric pumping is the slowness inherent in the molecular diffusion steps of heat and mass transfer in large scale equipment. One notable exception is pressure swing absorption which has cycle times on the order of a few seconds.
Another separation technique related to our method is field-flow fractionation (FFF) pioneered by J. C. Giddings and his coworkers for chromatographic separations. A field acts perpendicular to flow in a narrow channel, forcing particles toward a wall. Simultaneously, diffusion disperses the particles in the channel cross section. Particles least affected by the field and most affected by diffusion are swept downstream preferentially by the axial flow. Particles most affected by the field concentrate near the wall and move downstream slowly due to smaller axial velocities near the wall. A pulse of a mixture of particles will emerge in several fractions or peaks as in chromatography.
Giddings and others employed various external fields such as thermal, pressure gradient across membrane walls, magnetic, and centrifugal. A recent survey of FFF separation techniques was made by Janca (Janca, J., Field-Flow Fractionation: Analysis of Macromolecules and Particles, Marcel Dekker, Inc., New York (1988)). For the most part, work on FFF has been concentrated on the use of the technique as an analytical laboratory tool.
Several years ago one of the inventors herein developed a technique (U.S. Pat. No. 4,623,470 to Dr. Robert J. Adler) based on the concept of combining two factors: (1) a steady separative force which acts to produce a concentration gradient perpendicular to the walls of the containing tube, such as a steady centrifugal, magnetic, or electric field, and (2) periodic flow through the tube composed of cycles containing at least one step of forward flow and one step of backward flow, and usually also a step of zero flow, where the periodic flow and geometry are such as to interact to produce periodic local mixing, as for example flow through a curved tube which induces secondary flows perpendicular to the tube wall, or asymmetric flows through a straight tube which are periodically laminar and turbulent. The basic phenomena bringing about the separation are presumed to be migration of species perpendicular to the tube wall under the influence of the separative force which causes one or more species to concentrate along a solid surface in each cross section, and convective transport of particles, droplets, or molecules along the tube by the imposed periodic flows.
Thus, the method described above differs from parametric pumping in that it uses a steady separative force perpendicular to the tube axis rather than a periodic one or steady one varying along the tube axis, and specially modulated and controlled back-and-forth flows rather than symmetric periodic flows. The special back-and-forth flows bring about local periodic mixing and resuspension of the species being separated or fractionated, and also transport the species being separated along the conduit in which the process is being conducted. The local periodic mixing is enabled by a curved conduit, such as a helical tube, and/or flows which are asymmetric in their velocity. A species mixed or suspended in the fluid is transported by the motion of the fluid in one direction, and when the same species is adjacent to a solid boundary the other components are transported by the fluid in the opposite direction.
That method somewhat resembles sedimentation FFF, in which the flow channel is a helical tube within a centrifuge bowl, but it differs from FFF in that rapidly alternating rather than fundamentally unidirectional flows are used, and in that the separated products are produced essentially in a steady fashion at opposite ends of the apparatus, while in FFF the products are produced in an essentially sequential series of pulses at one end of the apparatus. Also, periodic secondary flows are employed advantageously as an essential part of the process, rather than being suppressed as is critical to FFF.
As claimed in the Adler patent, sedimentation is enhanced by centrifugal force about an axis of rotation of an elongated (preferably helical) chamber. For example, to separate a slurry into a concentrated fraction and a lean fraction, the basic process for a batch operation is as follows. Each cycle of periodic operation consists of three steps: step "a" of zero flow; step "b" of flow in a first direction through the helical tube; and a step "c" of flow in a second (opposite) direction through the helical tube. The steps are sequential and form a cycle which is repeated indefinitely to bring about the separation. Throughout all of the steps and cycles the helical tube spins steadily about its axis causing the slurry contained in the tube to be subjected to a constant centrifugal force field.
During step "a" the centrifugal force causes particles to move radially outward toward the outer wall of the helical tube, i.e. the portion of the tube wall farthest from the helical tube axis, where they become relatively concentrated.
In step "b", fluid is transported by pumping in a first direction through the helical tube. Generally the volume pumped is less than the volume of the helical tube. During the initial part of step "b" the particles continue to be located adjacent to the tube wall and are therefore temporarily immobilized with respect to axial flow. Thus the fluid moved along the helical tube by the pumping is relatively free of particles. As time passes within step "b", the axial flow begins to induce secondary flows or cross currents within the helical tube which resuspend the fine particles that had been concentrated along the outer wall of the tube. Step "b" is terminated when the developing secondary flows have resuspended the particles.
In step "c", the flow is reversed through the helical tube by pumping in the opposite direction. Generally the volume of fluid displaced is again at most the volume of the helical tube. Secondary flow again keeps the particles substantially resuspended so the particles are carried backwards with the fluid.
Separation occurs because the fluid moving in step "b" in a first direction is substantially free of particles while fluid moving in step "c" in the second (opposite) direction is relatively concentrated in particles. In other words, the particles lag behind the axial fluid motion in step "b", but move more nearly in phase with the axial fluid motion in the reverse direction in step "c". The repetition of cycles consisting of steps "a", "b" and "c" enable even a slight amount of separation in the cross sections to be cascaded into a large separation between the ends of the tube. Liquid accumulates free of fine particles in the reservoir adjacent the end of the helical tube toward which the flow is directed in step "b", and concentrated slurry accumulates in the reservoir adjacent the opposite end of the helical tube toward which the flow is directed in step "c".
Different sequences of steps with different step durations in the cycles enable slurries to be fractionated into portions according to particle size or density and enable emulsions or molecular mixtures to be separated or fractionated. This is because step "a" causes faster sedimenting particles preferentially to settle out, and they are left behind, relative to slower sedimenting particles, in step "b". They must then be resuspended by the flow in step "b" so that the repeated cycles will result in an accumulating separation.
The Adler patent also discusses (column 13, line 43 to column 14, line 9) the extension of the technique to a straight tube by making the forward and backward flows asymmetric in their flow rates. According to that disclosure, the fast flow should be the one that accomplishes the resuspension of particles, while the slower, reverse flow should not be a mixing one.
It has been found, however, that the process disclosed in the Adler patent for a straight tube was not adequately effective to separate the resuspension step from the reverse flow step; and that as a result the separation technique of the Adler patent is not as suitable for use with a straight tube. In fact, even in a curved tube, in which resuspension is to be achieved by secondary flows generated by the axial flows, resuspension is inadequate. That is, whether generated by secondary flow and/or turbulence in a curved tube or by turbulent flow in a straight one the resuspension mechanism of the Adler patent is not sufficiently vigorous nor easy to control. A need, therefore, existed for a way to adapt the Adler concept for use in any tube, and obtain better separations.